Interest Rate Risk - Narrative (Detail) (USD $) In Billions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2014	Dec. 31, 2013
Interest Rate Risk:
Derivative instruments, notional amount	$ 58.1	$ 52.4

Notional Amount Of Interest Rate Derivatives Settled

No definition available.

Name:	nly_NotionalAmountOfInterestRateDerivativesSettled
Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration